1. Field of the Invention
The present invention relates to a discharge-gas processing device that carries out dilution processing of off-gas discharged from a fuel cell.
The present application claims priority on Japanese Patent Application No. 2004-205747, filed Jul. 13, 2004, and Japanese Patent Application No. 2004-205748, filed Jul. 13, 2004, the contents of which are incorporated herein by reference.
2. Description of Related Art
As a type of fuel cell to be installed in a fuel cell vehicle and the like, a fuel cell is known in which electrical power is generated through chemical reactions of reactant gases. Among such fuel cells, a type of fuel cell is known that provides an anode and a cathode on opposite sides of a solid polymer electrolyte membrane, supplies a fuel gas (e.g., hydrogen gas) to the anode and supplies an oxidizing gas (e.g., air containing oxygen) to the cathode so that chemical energy produced in an oxidation reduction reaction of these reactant gases is directly extracted as electrical energy.
In this type of fuel cell, water is formed at the cathode side by the power generation, and a portion of the formed water permeates the solid polymer electrolyte membrane so as to diffuse into the anode area. In addition, a trace quantity of nitrogen contained in air supplied to the cathode permeates the solid polymer electrolyte membrane so as to diffuse into the anode side, where it mixes with the hydrogen gas. The power generation in the fuel cell may become unstable due to impurities such as the water and nitrogen on the anode side.
In particular, in a circulation-type fuel cell system in which an unreacted hydrogen gas (anode off-gas) that is discharged from the fuel cell is recycled and supplied to the fuel cell again after being mixed with fresh hydrogen gas in order to increase fuel efficiency, the concentration of the above-mentioned impurities at the anode side tends to gradually increase.
In order to solve such a problem in this type of fuel cell, the concentration of the impurities contained in the anode off-gas is reduced by periodically opening an exhaust valve to discharge anode off-gas containing the impurities from the anode off-gas circulation path in which the anode off-gas circulates.
When the anode off-gas discharged from the anode off-gas circulation path is to be discharged to outside (the atmosphere), a discharge-gas processing device dilutes the anode off-gas with a diluent gas (e.g., air as the cathode off-gas that is discharged from the cathode) to reduce the hydrogen concentration and then discharges it.
Japanese Unexamined Patent Application, First Publication No. 2004-127666 discloses an example of a conventional discharge-gas processing device. In this discharge-gas processing device, an anode off-gas introduction pipe is connected to a substantially rectangular box-shaped dilution container, a liquid reservoir portion that protrudes outward in a rectangle is formed in the center of the bottom of the dilution container, a cathode off-gas pipe is passed through the dilution container with a portion of the cathode off-gas pipe in the dilution container being disposed in the liquid reservoir portion, drain holes are provided in the portion of the cathode off-gas pipe housed in the liquid reservoir portion, and gas discharge holes are provided in the portion of the cathode off-gas pipe not housed in the liquid reservoir portion.
In the conventional discharge-gas processing device constituted in this way, the cathode off-gas discharged from the cathode of the fuel cell is always flowing through the cathode off-gas pipe during operation of the fuel cell. Meanwhile, when there is a discharge demand, anode off-gas is introduced into the dilution container via the anode off-gas introduction pipe and stagnates there. The anode off-gas that stagnates within the dilution container is gradually sucked into the cathode off-gas pipe through the gas discharge holes, and diluted by being mixed with the cathode off-gas that flows through the cathode off-gas pipe, thereby becoming a mixed gas that flows downstream through the cathode off-gas pipe.
Japanese Unexamined Patent Application, First Publication No. 2004-6183 discloses another conventional discharge-gas processing device. In this discharge-gas processing device, a meandering path is formed by providing a partition panel in the dilution container, thereby enabling introduction of an anode off-gas at the starting end portion of the path via an anode off-gas introduction pipe and enabling introduction of a cathode off-gas via a cathode off-gas branch pipe that branches off from a cathode off-gas pipe. The anode off-gas is diluted by mixing with the cathode off-gas while flowing through this path, with the diluted gas being discharged to the cathode off-gas pipe from the terminal end portion of the path, where it is diluted by being further mixed with the cathode off-gas.
However, in either of the above-mentioned discharge-gas processing devices, measures must be taken to ensure that the anode off-gas emitted from the anode off-gas introduction pipe to the dilution container does not blow-through the inside of the dilution container by the emission force to be sucked into the gas discharge holes of the cathode off-gas pipe.
In the case of the conventional discharge-gas processing device disclosed in Japanese Unexamined Patent Application, First Publication No. 2004-127666, making the gas discharge holes in the cathode off-gas pipe small as a measure for preventing the blow-through gives rise to the problem that the timing of discharging the anode off-gas from the anode off-gas circulation path must be delayed due to the longer stagnation time in the dilution tank (that is to say, a longer time is required to lower the hydrogen concentration).
Also, in the case of the conventional discharge-gas processing device disclosed in Japanese Unexamined Patent Application, First Publication No. 2004-6183, flow control is required for the cathode-off gas introduced into the dilution container as a measure to prevent the blow-through. However, it is difficult to carry out flow control simply with the size of the holes for introducing cathode off-gas. When the flow of the cathode off-gas becomes too small, even if blow-through is prevented by oversupply of cathode off-gas, the problem arises of inadequate dilution of the anode off-gas in the dilution container. In addition, the water content of the anode off-gas stagnates in the dilution chamber, leading to the possibility of impeding gas flow.
Furthermore, in the discharge-gas processing device in Japanese Unexamined Patent Application, First Publication No. 2004-127666, the internal pressure of the dilution container increases rapidly when anode off-gas is introduced and when the introduction amount of the cathode gas increases due to a rise in output of the fuel cell. Also, since there is hardly any pressure buildup in the dilution container when the anode off-gas is not being introduced, the dilution container enters the so-called breathing state and deforms.
In the case of having a liquid reservoir portion that protrudes outward in a rectangle at the bottom of a substantially rectangular box-shaped dilution container as in the case of the aforementioned conventional discharge-gas processing device, in order to ensure pressure resistance at the corner portions and ensure sufficient strength against deformation (repetitive stress) due to breathing of the dilution container, it has been necessary to reinforce the dilution container by, for example, providing ribs in all directions. However, adopting such a reinforced structure leads to various disadvantages such as the structure of the discharge-gas processing device becoming complicated, manufacturing difficulties, and weight increase.